Several previous vehicles operated on fixed guideways that do not use flanged railroad-style wheels have employed some sort of steering rails mounted at each side of the guideway. These vehicles have generally employed sets of rollers at each side that were selectively engaged with the steering rails to rotate in contact with the rails as a purely mechanical means of steering the vehicle. Because in such cases the steering rollers were required to be in constant contact with the steering rails, the vehicle speeds had to be limited to allow bearings of the steering rollers to turn at speeds that would enable acceptable bearing life. In addition, the purely mechanical nature of the vehicle steering generally made it necessary to provide some dead zone between sets of steering rollers that caused the vehicles to sometimes tended to lack the desired degree of smoothness, sometimes causing some discomfort to vehicle passengers.
In order to overcome the above problem, non-contact steering rail proximity sensors may be used that generate continuous electrical error signals that feed to vehicle steering controllers that drive steering servos to smoothly steer the vehicle to maintain each sensor a fixed distance from the reference steering rail. This approach provides enables each end of the vehicle to be smoothly steered to maintain the vehicle ends at a fixed distance from the side of the guideway. By selecting outputs from sensors at either the right or left side of the vehicle, the vehicle is caused to follow reference rails at either side of the vehicle. In this manner, the vehicle may be caused to steer from a main guideway to a side guideway or alternatively, from a side guideway to a main guideway. Selection of left or right side steering reference sensors can be accomplished either by an operator on board the vehicle or by an automatic vehicle control system.
However, vehicles operating on fixed guideways and steering automatically based upon sensing proximity to fixed steering reference rails of the guideway need a positive backup mechanical means for steering in the event that reference rail position sensing or automatic steering controls fail. Without such backup steering means, several adverse consequences can occur in the event of automatic steering failures. Steering failure can result in the vehicle crashing sideways into guideway components such as electric power supply rails located at the sides of the guideway and thereby causing damage to the guideway and vehicle. Steering failure can result in the vehicle running off of the guideway with potential for serious and even fatal consequences for vehicle passengers. In cases wherein the vehicle must follow either the left or right side steering rail during switching at switch “Y” points between guideways, absence of such backup mechanical steering means engaged on the same side as the steering reference rail could cause the vehicle to lose control and cause a serious crash. Such an event would be similar to that of a conventional railroad train attempting to pass through an open switch.